Motor Starting Current Calculator Formula

Understand the math behind the motor starting current calculator. Each variable explained with a worked example.

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Formulas Used

Full Load Current (FLA)

full_load_amps = fla

Starting (Inrush) Current

starting_current = fla * starting_multiplier

Starting kVA Demand

starting_kva = fla * starting_multiplier * sqrt(3) * voltage / 1000

Input Power

input_kw = watts / (efficiency_pct / 100) / 1000

Variables

VariableDescriptionDefault
motor_hpMotor Horsepower(HP)10
voltageOperating Voltage(V)480
efficiency_pctMotor Efficiency(%)90
power_factorPower Factor at Full Load0.85
starting_multiplierStarting Current Multiplier6
wattsDerived value= motor_hp * 746calculated
flaDerived value= watts / (sqrt(3) * voltage * efficiency_pct / 100 * power_factor)calculated

How It Works

Motor Starting Current

Electric motors draw significantly more current during startup than at full speed. This inrush can be 6-8 times the full-load current.

Formulas

FLA (3-phase) = HP x 746 / (sqrt(3) x V x Eff x PF)

Starting Current = FLA x Multiplier

Starting Methods and Multipliers

  • Direct-On-Line (DOL): 6-8x FLA
  • Star-Delta: 2-3x (reduced torque)
  • Soft Starter: 2-4x (adjustable)
  • VFD: 1-1.5x (best control)

    • Worked Example

    10 HP motor, 480 V 3-phase, 90% efficient, PF 0.85, DOL start (6x).

    motor_hp = 10voltage = 480efficiency_pct = 90power_factor = 0.85starting_multiplier = 6
    1. 01Motor watts: 10 x 746 = 7,460 W
    2. 02FLA: 7460 / (1.732 x 480 x 0.90 x 0.85) = 7460 / 635.6 = 11.7 A
    3. 03Starting current: 11.7 x 6 = 70.4 A
    4. 04Starting kVA: 70.4 x 1.732 x 480 / 1000 = 58.5 kVA

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